Impact strength modifiers have been used for a long time to improve the strength of adhesives against impact forces. In particular, generally epoxy resin compositions indeed have high mechanical strength but are very brittle, i.e., the cured epoxy resin fractures under impact forces such as occur, for example, in vehicle collisions, which results in the destruction of the bond.
For instance, two-component epoxy resin compositions cured at temperatures below 100° C. tend to embrittle and hence to exhibit a poor fracture behavior. Already a moderately strong impact loading causes the adhesive bond to fail.
It was suggested long ago to improve the impact strength of two-component epoxy resin compositions by using impact strength modifiers.
WO2006/128722 A1 pertains to impact strength modifiers for thermosetting epoxy adhesives. Described is a prepolymer based on a polyurethane having isocyanate terminal groups, said terminal groups being reacted with a protecting group compound that can, inter alia, also be amino compounds. Said compounds are described to be monofunctional and are no longer available as free functional groups after having reacted with the prepolymer.
U.S. Pat. No. 5,187,253 describes the reaction of an isocyanate prepolymer with a polyamine. However, the maximum molecular weight of the employed primary diamine is less than 600 g/mol. Further, U.S. Pat. No. 5,187,253 discloses the use of the prepolymer in two-component adhesive compositions.
EP 0 457 089 A2 describes the reaction of a polyetherpolyol with a diisocyanate and an at least difunctional amine that contains either at least two primary or secondary amino groups. The molecular weight range of the difunctional amine is stated as from 60 to 500 g/mol.
However, the polymers described here improve the impact strength of two-component room temperature curing epoxy resin compositions only to a relatively low degree. Still, they usually result in embrittlement of the produced bond after curing.
In particular, it has been found that the impact strength on electrogalvanized steel, an essential substrate in vehicle manufacturing, is often insufficient.